Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
When the engine is not running, some pressure may be bled through the canister to atmosphere, but the amount of this bleeding is limited to prevent escape of fuel vapors from the tank or canister into the atmosphere. Accordingly, pressure may build in the fuel tank. This is particularly the case with hybrid vehicles where the internal combustion engine is not always running and there are fewer opportunities to purge vapors.
When a vehicle operator refuels the vehicle, the fuel cap may be locked until venting is allowed to sufficiently reduce tank pressure. The inventors herein have developed systems and methods to handle depressurization during refueling events. In one example, a method comprises: before refueling a fuel tank having a pressure above a first predetermined pressure, releasing the pressure through a first valve to the predetermined pressure and then closing the first valve and opening a second valve to further reduce the pressure to a second predetermined pressure; and if flow through the second valve is less than desired, then opening the first valve until the second predetermined pressure is reached. Preferably the fuel cap is unlocked when the second predetermined pressure, typically around atmospheric pressure, is reached. In this way, depressurization is always achieved in a timely manner.
In another example, the method comprises: before refueling a fuel tank having a pressure above a first predetermined pressure, and when flow through a first and a second valve coupled to the fuel tank is at least at a desired flow, releasing the pressure through a first valve to the first predetermined pressure and then closing the first valve and opening a second valve to further reduce the pressure to a second predetermined pressure; and if flow through the first valve is less than desired, then opening the second valve until the first predetermined pressure is reached. Further, the second valve may be pulsed to gradually release pressure. In this way, the second valve may be advantageously used to release fuel tank pressure if flow through the first valve is less than desired.
In still another example, the method comprises: before refueling a fuel tank having a pressure below a first predetermined pressure, releasing the pressure through a second valve to a second predetermined pressure; and if the second valve is restricted and the pressure does not reach the second predetermined pressure within an expected time, then opening a first valve until the second predetermined pressure is reached, the first valve normally being used when the pressure is above the first predetermined pressure to reduce the pressure to the first predetermined pressure. In this way, prior to a refueling event, the fuel tank may be depressurized even if one of the two valves is diagnosed as having less than desired flow.
Accordingly, various methods are provided for cases where tank pressure is above a predetermined pressure, below the predetermined pressure, and even at negative pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.